So I thought I'd disabled it. However, I couldn't see any difference in some memory-intensive applications, so I decided to check if there was any performance event that would tell me how many prefetches were done. From the BKDG, I got PMCx067 Data Prefetcher. With the aid of libpfm4's check_event utility program, I got perf raw event 0x530267.

Then

$ perf stat -e r530267 /bin/ls > /dev/null

Performance counter stats for '/bin/ls':

28,457 r530267

I expected zero events. I resetted the MSR to 0 and reran perf, getting about the same count.